Call Changes practice software

There seems to have been a shortage of software to help people learn to follow Call Changes, which seems a shame.  So much of the mental work is mechanical, like doing arithmetic, and can be practiced and polished before coming to the tower if only there were a reliable way to present Call Changes “problems” and check answers.  So:

We have several mid-level ringers who need to learn or improve Call Changes.  Here is a web page, backed by a Javascript program, for that very purpose.